An area of continual focus for HPLC column improvement has been increased pH stability. Flexibility to operate a column over a wide pH range provides another dimension of control in the separation of ionizable compounds, a class which a vast majority of small molecule drug compounds fall into, and has become a valuable tool for chromatographers. By altering the pH of the mobile phase, the charge on many ionizable compounds can be altered, in turn altering the retention of the compound, hence the selectivity of the column. This allows “tuning” of the separation to meet specific requirements. As shown in FIG. 1, use of high pH for basic drugs in particular, can improve their retention and selectivity.
Column manufacturers, and academia, have spent great efforts to improve the pH stability of silica-based media. As unmodified silica is inherently unstable and begins to dissolve near and above pH 7, a multitude of surface chemistry processes like bonding, coating, and endcapping have been employed to “shield” the silica from the harmful effects of higher pH mobile phases with some degree of success. Other strategies have led to the exploration of polymer-based sorbents, which are stable over the entire pH range. However, polymer based HPLC sorbents suffer from poor physical morphology due to wide pore size distribution, numerous micropores, and low mechanical and structural stability when compared to their silica counterparts. These deficiencies generally result in mobile phase limitations, poor column and separation efficiencies, limiting their use in high-performance applications.
The following publications are considered to be related to the field of the present invention and are hereby expressly incorporated by reference in their entireties:                1. L. Sander and S. Wise, Synthesis and Characterization of Polymeric C18 Stationary Phases for Liquid Chromatography, Anal. Chem. 1984, 56, 504-510.G.        2. G. Schomburg et al. Immobilization of Stationary Liquids on Silica Particles by y-Radiation, Chromatographia Vol. 18, 5, 1984, 265-274        3. Y. Ohutsu et al., Structures and Chromatographic Characteristics of capsule-Type Silica Gels Coated with Hydrophobic Polymers in RPLC, Chromatographia Vol. 24, 1987, 380-384        4. G. Schomburg, Polymer Coating of Surfaces in Column Liquid Chromatography and Capillary Electrophoresis, Trends in Analytical Chemistry, Vol. 10, 5, 1991, 163-169        5. M. Hanson et al., Review. Polymer-coated Reversed Phase Packings in HPLC, Journal of Chromatography A, 656 (1993), 369-380        6. S. Kobayashi et al., Synthesis and Characterization of a Polymer-coated c18 Stationary Phases with High Carbon Content for LC, Journal of Chromatography A, 828 (1998), 75-81        7. N. Umeda et al., Synthesis of Multilayered Silica-based Hybrid Films from Difunctional Organosilanes by Co-Hydrolysis and Polycondensation with Tetraethoxysilane, Journal of Organometallic Chemistry, Vol. 686, 1-2, 2003, 223-227        8. D. Mochizuki et al., Molecular Manipulation of Two- and Three-Dimensional Silica Nanostructures by Alkoxysilylation of Layered Silacate Octosilicate and Subsequent Hydrolysis of Alkoxy Groups, Journal of ACS, 2005        
The following patents are likewise considered to be related to the field of the present invention and are hereby expressly incorporated by reference in their entireties: U.S. Pat. Nos. 4,539,061; 5,376,172; 6,261,357; 6,686,035; and WO 03/089106 A2.
Inorganic/Organic Hybrid media, sometimes referred to as “Hybrids” are composite materials, which incorporate both Inorganic and Organic components in order to provide advantageous properties not found in these materials individually. More recently, Hybrid materials have been explored for use as an HPLC medium with the hopes of bringing the best of both platforms to HPLC—the superior physiochemical morphology and mechanical strength of inorganic silica, and the pH stability and ionic inertness of organic polymers. One current HPLC column is disclosed in U.S. Pat. No. 6,686,035. This column is based on a porous “pure” first generation inorganic/organic hybrid particle formed by conventional sol-gel routes incorporating organic groups, namely methyl (—CH3), throughout its silica lattice base structure, and surface modified by bonding with varying alkyl silanes by conventional means. Incorporation of methyl organic groups yields some properties of a polymer-based media, providing resistance to degradation at high pH.
While this approach of a pure hybrid particle has yielded improved pH stability over many purely silica-based media, the technology has also introduced some polymer-like drawbacks such as lower efficiency, larger peak tailing, reduced mechanical strength, and higher backpressure as compared to silica-based media of similar particle and pore size. Some of the above-mentioned shortcomings, at least partially, can be attributed to wider pore size distributions compared to silica based media/phases as shown in FIG. 2a. Additionally, much of the organic component can remain entrapped and as a result unutilized (wasted) within the walls of the particle (interior) as opposed to being concentrated at the surface where hydrolysis first begins to occur during exposure to high pH. Essentially, these “entrapped” methyl groups will not contribute to reduced dissolution until media has already been damaged to the point of complete or significant loss of performance. At the same time, the presence of single-bond-attached (hanging) methyl groups, and correlated to them, isolated silanol groups in the fully coordinated silicon-oxygen lattice of the walls brings an element of heterogeneity to the otherwise homogenous silica gel structure that in turn results in reduced mechanical and structural stability of the whole particle. In addition, the abovementioned isolated silanol groups while being sterically hindered to an effective end-capping reaction may still actively interact with the small molecules of some analytes.
The '035 patent discloses another HPLC product. This product is based on a porous inorganic/organic hybrid particle formed by conventional sol-gel routes incorporating organic ethane (—CH2—CH2—) bridges throughout its silica lattice base structure, and surface derivatized with varying alkyl phases by conventional means. This approach of ethane bridged organic/inorganic hybrid particle dramatically alleviated some problems associated with the previously described product. The introduction of bridged chemistry that anchors an ethane molecule on both its sides to neighboring silicon atoms, improves overall structural strength (as opposed to the previous product) because of the absence of single-bond-attached (hanging) organic moiety and corresponding silanol groups. Nonetheless, this chemistry still carries an element of heterogeneity throughout the particle. There are still unutilized interior organic moieties scattered throughout the silicon-oxygen lattice that are away from a chromatographically active surface and consequently cannot contribute to its hydrolytic stability. Apart from the abovementioned, this new HPLC product has still somewhat higher backpressures, lower efficiencies, and larger peak tailing when compared to state-of-the-art silica based HPLC columns in the same category. This may again be attributed to a wider pore size distributions compared to silica-based media/phases as shown in FIG. 2b. 
Thus, there remains a need for a high-performance chromatographic media that retains the near ideal physical morphology and strength of silica, while providing pH stability and inertness to ionic interactions closer to that of a polymer.